Sulfur and Carbon Determination in Cast Irons

Carbon alloyed iron with carbon concentrations between 2 and 5% is known as cast iron, and is used for a variety of applications, such as cookware and barbecue grills, building parts or automotive parts such as cylinder blocks, cylinder heads or gearbox cases.

Cast irons can be found in a number of forms, with varied alloying elements and solidification constraints. The isolation of carbon in the form of cementite creates white cast iron, while the formation of graphite within the metal matrix results in gray cast iron.

On the whole, microstructure and alloying elements are the properties which affect cast irons. Thus, the concentration of the main alloying element, carbon, is vital for cast iron properties such as wear resistance, melting point, and strength.

Contamination of sulfur affects mechanical properties. Thus, the accurate regulation of carbon and sulfur content in cast irons is extremely important, and in fact vital in most cases. The combustion method is the most commonly chosen technique for the qualification and certification of carbon and sulfur concentrations in cast irons.

The combustion method uses elemental analysis to measure the carbon and sulfur contents to exceptionally precise levels, from mass percent levels right through to trace parts-per-million.

Elementar has used their century of experience in the elemental analysis of organic materials to establish a novel, forward-thinking and simple to use CS combustion analyzer for inorganic materials, namely, the inductar® CS cube, and the inductar® EL cube in CS mode, which operates identically.

The Inductar CS Cube

The inductar CS cube was created using Elementar's wide-ranging experience in automated sampling, efficient combustion processes, and gas detection techniques, and merges dependable data quality with greater user-friendliness.

The inductar CS cube comes with a long-lasting solid-state high-frequency induction furnace, allowing it to achieve sample temperatures of up to 2,000 °C, offering extremely accurate analysis of metallic and ceramic samples.

The device makes use of Elementar´s dependable autosampler, which can be used continuously, without supervision, and which has many years of demonstrable experience in day-to-day lab use with organic elemental analyzers. In addition to this, simple and rapid maintenance is a well-established benefit of all Elementar analyzers. Each part is labeled and can be changed in moments, without the need for tools.

Elementar has set up industry-leading standards for easy to operate instrument control, ensuring quick and intuitive inputting of data as well as fast analysis and reporting. Helpful apps, such as a step-by-step calibration wizard, help to direct the user through the process, while the software can also be adapted to permit communication with other lab equipment, as well as a simple connection to LIMS.

Figure 1 shows a diagram of the inductar CS cube.

Functional principle of the inductar CS cube.

Figure 1. Functional principle of the inductar CS cube.

Measuring Principle

In contrast to similar devices, a robotic arm is used to automatically slot the ceramic crucibles into the analyzer from the top. The ceramic cups are packed with samples made up of particles, chips or drillings. Additionally, the oxygen carrier gas current moves in a downward stream.

In this manner, a sheath gas flow is generated, which reduces the sticking of dust and fragments on the combustion tube to minimum levels. Moreover, the height of the ceramic crucible walls ensures minimum levels of sputtering of debris, resulting in clean combustion.

A programmable solid-state induction furnace is used to heat the samples to extremely hot temperatures. Following this, carbon and sulfur atoms mix with oxygen to become carbon monoxide, carbon dioxide, and sulfur dioxide.

A moisture trap and heated dust-trap cleanse the gas flow before it passes a wide-range IR sensor which calculates the levels of sulfur dioxide in the gas flow (Figure 2). Therefore, the sulfur dioxide levels measured provide the data to calculate the concentration of sulfur atoms within the sample.

Following this, an additional furnace is used to oxidize sulfur dioxide and carbon monoxide, forming sulfur trioxide and carbon dioxide. A sulfur trioxide trap is used to eliminate sulfur trioxide from the gas flow.

Finally, a further wide-range IR detector is used to measure the concentration of carbon dioxide and to thus, determine the total quantity of carbon atoms within the sample. On finishing, the ceramic crucible is automatically detached from the device.

The inductar CS cube meets the international standards ASTM E1019 on “Standard test methods for determination of carbon, sulfur, nitrogen, and oxygen in steel, iron, nickel, and cobalt alloys by various combustion and fusion techniques” and ISO 15350 on “Steel and iron – determination of total carbon and sulfur content – infrared absorption method after combustion in an induction furnace.”

Calibration

The inductar software offers an innate multi-point calibration with suitable reference materials. Multipoint calibration is applied instead of single-point calibration to take into account the effect of varying sample matrices on the calibration line.

In this illustration, a wide-range calibration was carried out using four certified reference materials (CRMs) of different carbon and sulfur concentrations. As a result, each of the CRMs was measured to the closest 1 mg into ceramic crucibles, adding roughly 2 gW/Sn and 0.5 g Fe accelerators of a constant and low blank value to each. These samples were examined two times by the inductar CS cube, using diverse sample amounts each time.

Figure 2 shows the measurement response, i.e., the concentrations of SO2 and CO2 indicators of the two IR detectors, shown over the analysis time. The software computes the calibration curve automatically by plotting the resultant peak areas alongside the total carbon and sulfur masses of the CRMs, followed by a linear deterioration (Figure 3).

CO2 and SO2 signals during the analysis.

Figure 2. CO2 and SO2 signals during the analysis.

Linear wide-range calibration curves for carbon and sulfur.

Figure 3. Linear wide-range calibration curves for carbon and sulfur.

It is not necessary to perform calibration on a daily basis. The default calibration curve can be used for a long period of time, while minor daily deviations can be managed by applying a daily factor.

Analysis of Cast Iron

A number of cast irons were examined to demonstrate the performance of the inductar CS cube, with the results shown in Table 1. The ceramic crucibles were warmed at 1,100 °C for 60 minutes in an ambient atmosphere – something which is particularly vital for the analyses of low carbon concentrations. A sanitary crucible tong was used for additional handling.

Cast iron samples between 0.25 g and 0.50 g were measured to the closest 1 mg into the warmed ceramic crucibles, following by around 2 gW/Sn and 0.5 g Fe accelerators to assist with the combustion. The crucibles were positioned in the autosampler and analyses were carried out.

Ahead of the analysis, the sample mass was automatically moved from the balance into the software. The software used equations 1 and 2 to determine the concentrations of carbon and sulfur. Equation 1 is the calibration line, determined from the linear relation between the absolute carbon and sulfur mass of certified standards and the peak area in Figure 3.

     Eq (1) y = s · x + b
     Eq (2) c = (y - b) / (s · m)

The concentration (c) is worked out using equation 2 and represents the role of the sample mass (m), peak area (y), the slope of the calibration line (s) and value b, which is where the calibration line and y-axis meet.

Table 1. Analyses of the carbon and sulfur content in different materials using the inductar CS cube.

Analyses of the carbon and sulfur content in different materials using the inductar CS cube.

Summary and Discussion

The combustion method has been applied effectively to calculate concentrations of carbon and sulfur in a number of cast irons. Outstanding linearity with a multi-point calibration offers a precise analysis of carbon concentrations up to 5 %.

The inductar CS cube elemental analyzer, thanks to its minimal requirements for the uniformity of samples, as well as its quick and precise results, is ideally suited for process control, quality control or for the certification of cast irons. The inductar CS cube and inductar EL cube in CS mode meet the international standards ASTM E1019 and ISO 15350.

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This information has been sourced, reviewed and adapted from materials provided by Elementar Analysensysteme GmbH.

For more information on this source, please visit Elementar Analysensysteme GmbH.

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